A. Technical Field
The present invention relates to respective improvements of a one-liquid low temperature hardenable type colored enamel paint and a one-liquid low temperature hardenable type clear paint, which are used for coating of car bumpers and so on, because the paints have excellent durability and waterproofing and further have softness. Colored enamel paint is mainly coated onto plastic materials such as bumpers, and the clear paint is mainly used for overcoating of a base coat layer, such as a coat layer of the colored enamel paint, as formed on a material surface.
B. Background Art
In recent years, there is a problem of acid rain as brought about by environmental contamination. Solution of this problem is especially important to cars running outdoors, and it is demanded that car paints as used for plastic materials of bumpers and so on and for steel sheets of bodies and so on all should be excellent in the acidproofing, and that coating films of the paints should not deteriorate even if acid rain adheres to their surfaces.
At present, the most commonly used car paints are those which harden by melamine-crosslinking. However, it is difficult to say that these paints have sufficient acidproofing, because the melamine resin decomposes by the action of an acid.
As to the car bumper paint, a coating film resultant therefrom needs to be soft for functions of bumpers, so the glass transition temperature of the coating film is usually set to be low. Thus, there are also problems in that when compared with the steel sheet paint, the bumper paint is easier to contaminate and more difficult to remove the contamination from. There is a possible method for improving the contaminationproofing of the bumper paint, in which the glass transition temperature of the coating film is enhanced to harden the coating film. However, such a method causes another problem in that the softness, namely, the flexibility, is so much decreased that the coating film cracks. Thus, as to the bumper paint, the contaminationproofing and the flexibility are mutually exclusive properties, and therefore it is desired to attain both properties.
Japanese Allowable Patent Publication (Kokoku) No. 4-508 discloses a coating composition, comprising an acrylic resin, a polyalkylene glycol resin, an amino resin, and a blocked isocyanate, as a paint that satisfies basic performance of bumper paints. This coating composition is assumed to be capable of forming a coating film that is excellent in properties such as weatherproofing and waterproofing and has good hardness due to the combination of the polyalkylene glycol resin with the acrylic resin. However, this coating composition is not sufficient with regard to the contaminationproofing or the acidproofing. In this case, the polyalkylene glycol resin, which acts as a softener and is included in the coating composition as an essential component thereof, is considered to decrease the contaminationproofing and the acidproofing.
Japanese Patent Application Publication (Kokai) No. 1-158079 discloses a coating composition, which comprises an acrylic copolymer and a melamine resin and may further comprise a blocked isocyanate, as a bumper paint. This paint is hardened by baking at a high temperature. For car usage, thus, a method, in which a paint is coated and then the resultant coating film is hardened by baking at a high temperature, is carried out, but where materials for bumpers are plastic materials, it is demanded to bake the coating film at a low temperature of about 120.degree. C. because the plastic materials merely have low heatproofing. However, where the conventional paint as mentioned immediately above is baked at a low temperature, the hardening is insufficient, so that the contaminationproofing and the acidproofing are both low.
Although used for metals, a paint for metals of cars is disclosed in Japanese Patent Application Publication (Kokai) No. 4-246483. This paint comprises an organic tin monocarboxylate as a blocked isocyanate dissociation catalyst. In addition, Japanese Patent Application Publication (Kokai) No. 7-258599 discloses a steel sheet paint comprising an inorganic catalyst as a blocked isocyanate dissociation catalyst. As to this paint, the inorganic catalyst is insoluble in solvents, so this paint cannot be used as a clear paint for which transparency is demanded. Similarly to the above-mentioned conventional bumper paint, where baked at a low temperature, these steel sheet paints also merely insufficiently harden and therefore provide inferior results with regard to the contaminationproofing and the acidproofing.
Thus, none of the prior arts provides a paint that sufficiently hardens even by baking at a low temperature and secures the contaminationproofing and the acidproofing. From such circumstances, it is hoped to develop a car paint that sufficiently hardens by baking at a low temperature and is excellent in the contaminationproofing and the acidproofing.